Article 13. 1 Drug effects on the nervous system

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Article 14.2 Sense Organ

A sense organ is any collection of cells in an organism that responds to information about certain changes in the organism's internal or external environment. Each sense organ reacts to a particular type of stimulus. The sense organ converts the stimulus into a nerve impulse which is sent to the organism's brain to be processed and identified. Human sense organs are the eyes, ears, nose, mouth, and skin--each having its own particular type of receptors.

Sensory Receptors

Although there is great variety in how different animals experience the world through their senses, they all have sensing systems that operate by using sensory receptors. Thus, all sense organs, no matter what they detect, have and need sensory receptors. A receptor is a group, or cluster, of nerve cells that react to a particular stimulus and receive information.

Sensory receptors make-up the most familiar sense organs, such as the ears and eyes. These receptors can be classified according to the type of energy or stimulus to which they respond. Chemoreceptors (for taste and smell) respond to certain chemical compounds. Mechanoreceptors (for touch) respond to mechanical energy. Auditory receptors (for hearing) respond to sound wave vibrations, although some consider them to be a form of mechanoreceptor since the pressure waves of sound are a real, physical force. Photoreceptors (for sight) respond to light energy.

These four types of receptors correspond to the five human senses (taste and smell both use chemoreceptors) and, therefore, to our five major sense organs. Another important type of receptor is known as the proprioceptors. These are sensory receptors located inside an animal's muscles and joints. Proprioceptors allow the animal to be aware of its entire body position, as well as to keep its balance. It is because of this sense that a person is able to dress in complete darkness.

The human body has two senses that employ chemoreceptors--taste (gustation) and smell (olfaction). The first uses taste buds (mainly in the tongue) as the receptors for dissolved chemicals. The second uses olfactory epithelium ("smelling skin") in the nasal cavities to detect airborne chemicals. The sense of touch is located in the skin. Here, mechanoreceptors are activated when they change shape by being pushed or pulled. Auditory, or hearing, receptors are located in the cochlea, deep within the ear. They detect or respond to pressure waves (since sound is actually a vibration of the air). Finally, the rods and cones in the retina of the eyes are the photoreceptors that enable people to see.

Humans would have none of their five senses if they did not also have a vast, branching network of nerves. These nerves take all of the coded messages to the brain, which translates or interprets them. The brain tells a person exactly what he or she is sensing. As a result, people do not really see with the sense organs called the eyes, for it is the brain that interprets the signals the eyes are sending. The brain tells a person that he or she is seeing a beautiful rainbow and not a dangerous fire. The role of the sense organs then is only to gather the information. Understanding that information is left to the brain.

Source Citation: "Sense Organ." U*X*L Complete Life Science Resource. Ed. Leonard C. Bruno and Julie Carnagie. Detroit: U*X*L, 2001. Science Resource Center. Thomson Gale. 25 February 2007

____ 8. Use Article 14.2 to answer the following question.

Pressure sensors in the skin respond when they are physically deformed, making them









____ 9. Use Article 14.2 to answer the following question.

A large portion of the human brain processes information from rods and cones. This shows the importance of the sense of









Article 15.1 Homeostatic Mechanisms

Homeostatic mechanisms control a property of all living things called homeostasis. Homeostasis is a built-in, automated, and essential property of living systems. Breathing is an example of a homeostatic property. Homeostatic mechanisms are self-regulating mechanisms that function to keep a system in the steady state needed for survival. These mechanisms counteract the influences that drive physiological properties towards a more unbalanced state.

The mechanisms that regulate homeostasis operate by feedback mechanisms. Negative and positive feedback mechanisms operate in living things. Negative feedback mechanisms reverse the direction of the change. This maintains the constant, steady state and so represents homeostasis. Positive feedback, on the other hand, acts to change the variable even more in the direction in which it is changing. Thus, positive feedback is not a homeostatic mechanism.

In a healthy body, homeostatic mechanisms operate automatically at different levels; molecular, cellular, and at the level of the whole organism. At the molecular level, the activity controlled by one gene can be under regulatory control by another gene. At the cellular level, a well-studied homeostatic mechanism is contact inhibition, in which cells stop dividing when they begin to crowd in on each other. Cancer, in which a hallmark is the rampant growth and division of cells, is a condition where the homeostatic mechanism of contact inhibition is inoperative or defective. At the whole organism level, a homeostatic mechanism is a vital part of birth.

The importance of homeostatic mechanisms to the well being of an organism is underscored by the consequences of their failure. For example, at body temperatures of 107°F (42°C), the negative feedback systems cease to function. The high temperature then acts to speed up the body's chemistry, raising temperature even more. This, in turn, further accelerates body chemistry, causing a further rise in temperature. This cycle of positive feedback is lethal if not halted.

Source Citation: Hoyle, Bryan. "Homeostatic mechanisms." World of Anatomy and Physiology. Ed. K. Lee Lerner and Brenda Wilmoth Lerner. Detroit: Gale, 2002. Science Resource Center. Thomson Gale. 25 February 2007

____ 10. Use Article 15.1 to answer the following question.

The best definition of the term “homeostasis” is


automatic changes.




never changing.


steady state.

Article 15.2 Endocrine System

The endocrine system is the human body's network of nine glands and over 100 hormones that maintain and regulate numerous events throughout the body. The glands of the endocrine system include the pituitary, thyroid, parathyroids, thymus, pancreas, pineal, adrenals, and ovaries or testes; in addition, the hypothalamus, in the brain, regulates the release of pituitary hormones. Each of these glands secretes hormones (chemical messengers) into the blood stream. Once hormones enter the blood, they travel throughout the body and are detected by receptors that recognize specific hormones. These receptors exist on target cells and organs. Once a target site is bound by a particular hormone, a cascade of cellular events follows that culminates in the physiological response to a particular hormone.

The pituitary gland has long been called the master gland because it secretes multiple hormones that, in turn, trigger the release of other hormones from other endocrine sites. The pituitary is roughly situated behind the nose and is anatomically separated into two distinct lobes, the anterior pituitary (AP) and the posterior pituitary (PP). The entire pituitary hangs by a thin piece of tissue, called the pituitary stalk, beneath the hypothalamus in the brain. The AP and PP are sometimes called the adenohypophysis and neurohypophysis, respectively.

The thyroid is a butterfly-shaped gland that wraps around the back of the esophagus. The major hormones produced by the thyroid are triiodothyronine (T3), thyroxine (T4), and calcitonin. T3 and T4 are iodine-rich molecules that fuel metabolism. The thyroid hormones play several important roles in growth, metabolism, and development. The thyroids of pregnant women often become enlarged in late pregnancy to accommodate metabolic requirements of both the woman and the fetus.

Thyroid hormones accelerate metabolism in several ways. They promote normal growth of bones and increase growth hormone output. They increase the rate of lipid synthesis and mobilization. They increase cardiac output by increasing rate and strength of heart contractions. They can increase respiration, the number of red blood cells in the circulation, and the amount of oxygen carried in the blood. In addition, they promote normal nervous system development including nerve branching.

While most people have four small parathyroid glands poised around the thyroid gland, about 14% of the population have one or two additional parathyroid glands. Because these oval glands are so small, the additional space occupied by extra glands does not seem to be a problem. The sole function of these glands is to regulate calcium levels in the body. Although this may seem like a simple task, the maintenance of specific calcium levels is critical. Calcium has numerous important bodily functions. Calcium makes up 2 to 3% of adult weight with roughly 99% of the calcium in bones. Calcium also plays a pivotal role in muscle contraction and neurotransmitter secretion.

In young children, the thymus extends into the neck and the chest, but after puberty, it begins to shrink. The size of the thymus in most adults is very small. Like some other endocrine glands, the thymus has two lobes connected by a stalk. The thymus secretes several hormones that promote the maturation of different cells of the immune system in young children. In addition, the thymus oversees the development and education of a particular type of immune system cell called a T lymphocyte, or T cell.

Source Citation: "Endocrine system." World of Health. Ed. Brigham Narins. Online. Detroit: Thomson Gale, 2007. Science Resource Center. Thomson Gale. 27 February 2007

____ 11. Use Article 15.2 to answer the following question.

The pituitary gland is called the master gland because it


is controlled by the hypothalamus.


secretes more hormones that any other gland.


secretes hormones that make other glands release hormones.


is the one gland a person can not live without.

____ 12. Use Article 15.2 to answer the following question.

Metabolism is controlled by hormones secreted by the


adrenal glands.


thymus gland.


parathyroid glands.


thyroid gland.

Scenario 15.1 Homeostatic adjustments

The body adjusts for increased water intake by increasing urine output. Conversely, it adjusts for increased water loss or decreased water intake by reducing urine output. These homeostatic adjustments involve the nervous system and two different hormones of the endocrine system, antidiuretic hormone (ADH) and aldosterone. Nelson, Biology 30, p 490.

____ 13. Use Scenario 15.1 to answer the following question.

What is the source of aldosterone?


adrenal glands




pituitary gland


thyroid gland

Article 16.1 Puberty

Puberty is the period of sexual maturity when sexual organs mature and secondary sexual characteristics develop. Puberty is also the second major growth period of life--the first being infancy. A number of hormones under the control of the hypothalamus, pituitary, ovaries, and testes regulate this period of sexual growth, which begins for most boys and girls between the ages of nine and 15. The initial obvious sign of female puberty is the beginning of breast development, whereas the initial obvious sign in males is testicular enlargement. Since early signs of female puberty are more noticeable, it is sometimes assumed that female puberty precedes male puberty by quite a bit. However, males usually start puberty just a few months after females, on average. In males, puberty is marked by testicle and penile enlargement, larynx enlargement, pubic hair growth, and considerable growth in body height and weight. In females, puberty is marked by hip and breast development, uterine development, pubic hair growth, menstruation, and increases in body height and weight. Because of the extensive growth that occurs at this time, a balanced, nutritious diet with sufficient calories is important for optimal growth. Although puberty was originally used to classify the initial phase of early fertility, the term is also used to include the development and growth which culminates in fertility. In this sense, puberty usually lasts two to five years and is accompanied by the psychological and emotional characteristics called adolescence.

Physical maturity

Puberty marks the physical transition from childhood to adulthood. While the changes that accompany this time are significant, their onset, rate, and duration vary from person to person. In general, these changes are either sexual or growth related. The pubertal growth spurt is characteristic of primates. Although other mammals may have increased reproductive organ growth, their overall size does not increase as dramatically. The major control center for human pubertal development is the hypothalamus for both sexes, but puberty is accompanied by additional growth of the adrenal glands, as well. The added adrenal tissue secretes the sex hormones, androgens or estrogens, at low levels. The adrenal sex hormones are thought to initiate the growth of pubic and axillary (under-arm) hair. This adrenal maturation is called adrenarche.

It is not known exactly what triggers puberty to begin. However, the hypothalamus sends out gonadotropin hormones responsible for sperm and egg maturation. One theory holds that normal brain growth towards the end of childhood includes significant hypothalamic changes. Hypothalamic receptors are thought to become more sensitive to low levels of circulating sex steroids. These changes enable the neuroendocrine system to initiate spermarche (sperm maturation) and menstruation in puberty. However, these early hormonal fluctuations begin at night and remain a nocturnal pulse for some time before they are detectable while awake. Some behavioral changes are related to pubertal hormonal changes, as well. The increase in testosterone is associated with more aggressive behavior in males. And libido (sex drive) increases occur for some teenagers in association with estrogen and testosterone increases. These effects are also carried out through sex hormone receptors on the hypothalamus.

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